136 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



mizes that of the monodelph orders. The dentition 

 of the carnivorous forms is creodont ; that of the kan- 

 garoos is perissodactyle, and that of the wombats is 

 rodent. Other forms repeat the Insectivora. I there- 

 fore consider the placental series especially. I have 

 already shown that the greater number of the types of 

 this series have derived the characters of their molar 

 teeth from the stages of the following succession. 

 First, a simple cone or reptilian crown, alternating 

 with that of the other jaw. Second, a cone with an- 

 terior and posterior lateral denticles. Third, the den- 

 ticles rotated to the inner side of the crown below, and 

 outer side above forming with the principal (median) 

 cone a three-sided prism, with tritubercular apex, 

 which alternates with that of the opposite jaw. Fourth, 

 development of a heel projecting from the posterior 

 base of the lower jaw, which, in mastication, meets 

 the crown of the superior, forming a tubercular-sec- 

 torial inferior molar. From this stage the carnivorous 

 and sectorial dentition is derived, the tritubercular 

 type being retained. Fifth, the development of a pos- 

 terior inner cusp in the superior molar, and the eleva- 

 tion of the heel in the inferior molar, with the loss of 

 the anterior inner cusp. Thus the molars become qua- 

 dritubercular, and opposite. This is the type of many 

 of the Taxeopoda, including the Quadrumana and In- 

 sectivora as well as the inferior Diplarthra. The higher 

 Taxeopoda (Hyracoidea) and Diplarthra add various 

 complexities. Thus the tubercles become flattened 

 and then concave, so as to form V's in the section pro- 

 duced by wearing ; or they are joined by cross-folds, 

 forming various patterns, of which the most special- 

 ized is that of the horse. In the Proboscidia the latter 



